This application claims priority of International Application No. PCT/EP01/01664, filed Feb. 15, 2001 and German Application No. 100 07 201.1, filed Feb. 17, 2000, the complete disclosures of which are hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a method for the synchronized control of a plurality of stepping motors SM1, SM2 . . . SMn which serve as drives in a feed system, wherein the stepping motors SM1, SM2 . . . SMn are controlled at a base frequency fa and a cycle of the base frequency fa is proportional to a given step length SL.
b) Description of the Related Art
It is known from the prior art to transform the predetermined step angle defined for a stepping motor into increments of a longitudinal movement by controlling and, on this basis, to operate positioning systems by means of which displacements of device component groups along an adjustment area or a path are carried out with high accuracy.
With a suitable arrangement of the coils and phase control of the stepping motor, a sensitive control can be achieved and, therefore, the rotational movement can be advanced in step angles which are so small that even in optical precision instruments, e.g., microscopes with zoom devices, the precise positioning of the individual zoom groups required for changing the magnification while retaining imaging sharpness is achieved.
In the current state of development, typical step angles are 3.75xc2x0 in permanently excited stepping motors, 1.8xc2x0 in hybrid stepping motors and 1xc2x0 in variable reluctance stepping motors. Generally, spindle systems are used to transform the rotational movement into a longitudinal movement.
Positioning systems in which a component group to be displaced along a path is associated with each stepping motor have been developed for applications in zoom objectives as well as for other applications. Particularly in applications for zoom objectives, there is a need to displace the different component groups at different speeds so that, after a displacement, the zoom groups are at a predetermined distance relative to one another, which distance must be sufficient for the newly adjusted magnification and is also required for retaining the imaging sharpness. An xe2x80x9carrangement for direct control of the movement of a zoom system in a stereo microscopexe2x80x9d which works on this principle in described, for example, in WO 99/60436.
In the prior art, when controlling a plurality of stepping motors belonging to a positioning system, a counter unit or timer unit is associated with each stepping motor and each path. All counter units and timer units are loaded with different frequencies and times which relate to an associated path and were calculated based on the corresponding positioning task.
The stepping motors are started together and, at the conclusion of a predetermined sequence of cycles of the base frequency or control frequency, the displacement is initiated via output units, wherein an output unit is always associated with a stepping motor. For a further, subsequent adjustment, the counter units or timer units are reloaded and started in corresponding manner.
This sequence is repeated for every new positioning preset and the component groups reach the predetermined destinations on their path each time. In so doing, the component groups are displaced over different path lengths at the same times, i.e., they achieve their target positions at different speeds which are achieved in that the individual stepping motors are controlled with different frequencies.
Since every path also has separate acceleration profiles, particularly for the displacement of zoom groups, a positioning system constructed in this way requires a relatively large storage capacity and, consequently, a rather long calculating time.
In addition, because of the high feed speed that is requiredxe2x80x94in the present case, the maximum path deviation should be no more than 10 xcexcm in all drivesxe2x80x94every counter unit or timer unit comprises at least three individual counters.
In order to achieve the maximum possible adjustment speed, the stepping motor that must carry out the greatest displacement on the path associated with it is considered as the leading drive, to which all other drives which are assigned shorter feed distances are to be oriented in that they are operated more slowly.
It is clear from the description above that the relatively high expenditure on storage capacity and calculating time is due to the fact that a separate frequency is provided for each individual stepping motor of a positioning system of this type.
On this basis, it is the primary object of the invention to further develop a method of the type described in the beginning in such a way that it is possible to control the entire positioning system in a more economical manner while retaining a high positioning accuracy and optimal adjustment speed.
According to the invention, different step lengths SL1, SL2 . . . SLn are triggered with each cycle of the same base frequency fa in the individual stepping motors SM1, SM2 . . . SMn, wherein a specific step length SL1 is associated with stepping motor SM1, a specific step length SL2 is associated with stepping motor SM2, and so forth.
Accordingly, it is no longer required to provide separate control frequencies for every stepping motor. All stepping motors can now be controlled at the same frequency and only a counter unit or timer unit is required for all stepping motors of the positioning system.
Since every stepping motor travels over the specific step length SL1, SL2 . . . SLn assigned to it when controlling with one and the same frequency, it is achieved in a simple manner that the individual stepping motors SM1, SM2 . . . SMn move at different speeds and, at the conclusion of an adjusting movement, each of the component groups to be displaced has traveled over different displacement distances and has reached its target position.
In a preferred construction of the invention, stepping motor SM1, for example, is operated at a step length SL1 which corresponds to the whole step of the stepping motor SM1 during a complete rotating field rotation. The stepping motor SM1 is coupled with a component group which must travel a greater displacement distance than the other component groups when a positioning command is initiated.
Smaller step lengths SL2 . . . SLn corresponding to a partial rotating field rotation are assigned to the rest of the stepping motors SM2 . . . SMn. Accordingly, the component groups coupled with these stepping motors SM2 . . . SMn travel shorter displacement distances.
The whole step length SM1 resulting from a complete rotating field rotation is divided into a plurality of partial steps, or microsteps, as they are called, based on the fact that every position of the torque vector within a rotating field rotation is adjustable in theory by adapted control of the currents to be impressed in the individual phases. This type of control is known per se and described for example in Schxc3x6rlin, F., xe2x80x9cControlling, regulating and driving with stepping motors [Mit Schrittmotoren steuern, regeln und antreiben]xe2x80x9d, Fransisverlag 1996, second edition, 1996.
A quantity z of discrete microsteps can be generated in this way. For practical use of this effect, division of a whole step into z=100 microsteps is sufficient and is a proven basis for the configuration of positioning systems.
In this respect, one construction of the invention consists in that SL1=k1/z, SL2=k2/z . . . SLn=kn/z is true for step lengths SL2 . . . SLn, where k1 to kn are equivalents of the microsteps to be predetermined per cycle of the base frequency fa.
Assuming that a whole step length corresponding to a complete rotating field rotation is assigned to stepping motor SM1, the equivalence k1=100 is true for stepping motor SM1 when z=100. Consequently, when k1=100, the specific step length SL1 of stepping motor SM1 is 100 microsteps or a whole step, i.e., a displacement distance corresponding to a whole step length or 100 microsteps is initiated at every control pulse or every cycle of the base frequency fa.
For example, if the equivalence kn=25 is assigned to stepping motor SMn, 25/100 microsteps would be initiated via the stepping motor SM2 while controlling at the same base frequency fa per control pulse 25/100. Consequently, the speed of stepping motor SM1 would be higher than the speed of stepping motor SMn by a factor of four.
Analogously, this is also true for the rest of the stepping motors SM2 . . . SMnxe2x88x921 of the same positioning system when equivalents k2 . . . Knxe2x88x921 are assigned to them corresponding to the provided adjustment distance of the respective coupled component groups. In this way, all stepping motors SM1, SM2 . . . SMn move at different speeds when controlling at the same base frequency fa and reach the predetermined target positions simultaneously at the end of a predetermined sequence of cycles.
In a particularly preferred construction of the invention, characteristic feed positions, e.g., starting positions, reversing positions and/or stopping positions, are assigned to the individual stepping motors SM1, SM2 . . . SMn or to the component groups to be displaced on the corresponding path, and the step lengths SL1, SL2 . . . SLn are predetermined by equivalents k1, k2 . . . kn, depending on the distances between characteristic feed positions on a path, in such a way that the displacement between two feed positions is initiated when all stepping motors SM1, SM2 . . . SMn are controlled with the same quantity of cycles of the base frequency fa.
The invention will be described more fully in the following with reference to an embodiment example.